Neither Cruz nor Rubio’s negatives are anywhere as high as Trump’s. What makes you think that blue-collar Reagan Democrats won’t go for Cruz given his background?
Cruz is calling for privatizing social security, etc. far more extreme than Reagan, not even comparable. Cruz is more like Goldwater. Plus, Reagan was attractive due to his willingness to fight the USSR and supported strong national security (on nsa spying, etc.), Cruz is closer to Rand’s isolationism and national security positions.